Evolution of the full energy structure of Mn⁴⁺ in fluoride phosphors under high pressure conditions

This paper analyzes the photoluminescence excitation and emission spectra of fluoride phosphors doped with Mn⁴⁺: KNaSiF₆:Mn⁴⁺, Rb₂GeF₆:Mn⁴⁺, and Na₃HTiF₈:Mn⁴⁺ under high pressure conditions. From the optical spectra, the pressure-dependent energies of optically active ⁴T₂, ⁴T₁, and ²E crystal field subterms of Mn⁴⁺ have been determined in the 0-30 GPa pressure range. A strong blueshift of the ⁴T₂ and ⁴T₁ subterms was found, as expected from the Tanabe-Sugano diagram for Mn⁴⁺ (d³). At the same time, the ²E emitting state exhibited a redshift under pressure - an effect opposite to the prediction of the Tanabe-Sugano diagram. This is a manifestation of the pressure-driven nephelauxetic effect, governed by pressure induced changes of Racah parameters, which demonstrates the necessity of taking into account the Racah parameters for a correct description of Mn⁴⁺ emission under pressure. The high pressure experimental data allowed to determine the pressure dependence of crystal field strength parameter Dq and Racah parameters B and C. Finally, obtaining the pressure dependence of Dq and Racah parameters allowed to calculate the full energy structure of the d³ configuration of Mn⁴⁺ in KNaSiF₆, Rb₂GeF₆, and Na₃HTiF₈ in the pressure range of 0-30 GPa. The calculations reproduced the redshift of the ²E emitting state under pressure, as well as gave the pressure shift direction and magnitude for all crystal field subterms of Mn⁴⁺ up to 50 000 cm^-1 (i.e. the equivalent of the Tanabe-Sugano diagram for high-pressure experiments). The approach presented in this paper can be easily extended for calculating the energy structure of materials doped with isoelectronic Cr³⁺ as well as other transition metal ions.